Pollutant emissions estimation in road transport models

The report intends to analyse the problem regarding the combined use of transport and emissions models, to understand the impact on emissions due to mobility. The first part of the work is an overview of the transport and emission models carried out until now. The developments of these models have always followed separate routes and this fact explains the current difficulty using these models in a joined way. This is due also to the different quality of needed data: more aggregate for the transport models and very disaggregate for the emission models. Then, the core of the work deals with a comprehensive analysis of the combined use of transport and emissions models, emphasising the problems and lacks affecting these ones and giving some critical remarks. The main findings concern the estimation of the speed, vehicle miles travelled and operating modes, and capacity of the road. In addition, another important issue involves inconsistencies in the way the stepwise travel demand modelling process can be implemented, to understand how different aggregation schemes might affect the results of modelling. The importance of the reliability of transport data is largely demonstrated in this work. It was found that the emission factors calculated by the model vary substantially when the travel-related inputs to the model are varied within the usual ranges of accuracy and precision expected with the current state of the practice in transport. Another important element studied is the reliability of emissions models, as average as modal (or instantaneous). Actually, the average emissions model do not enable to reliably evaluate operational improvements that smooth traffic flow such as ramp metering, signal co-ordination and many ITS strategies. The “modal models” permit to enhance the performance of these ones, but the use of this kind of tool requires the input data mentioned above that need improved transport models. In addition, the uncertainty of the results is still a problem. At last, some researches and relative applications have been presented to understand the possible approaches followed to improve the actual estimations of emissions. The analysis of these efforts shows that an integrated approach is still far and each researcher tends to improve some particular aspects, as the activity based framework, the assignment phase and the disaggregation of transport data. At the end, a proposal is presented to manage the actual needs of precision under the viewpoint of “sample enumeration” technique. This is a possible approach to overcome actual difficulties and it indicates a possible way for the future research.